9. An overall appraisal of the health and psychological effects of cannabis

9.3 A comparative appraisal of health risks: alcohol, tobacco and cannabis use

The probable and possible adverse health and psychological effects of cannabis need to be placed in comparative perspective to be fully appreciated. A useful standard for such a comparison is what is known about the health effects of alcohol and tobacco, two other widely used psychoactive drugs. Cannabis shares with tobacco, smoking as the usual route of administration, and resembles alcohol in being used for its intoxicating and euphoriant effects.

Considerable care must be exercised in making such comparisons. Firstly, the quantitative risks of tobacco and alcohol use are much better known than the health risks of cannabis, since alcohol and tobacco have been consumed by substantial proportions of the population, and there have been 40 years of scientific studies of the health consequences of their use. Cannabis, by contrast, has been much less widely used, and for a shorter period, in Western society; it has been primarily used by healthy young adults, and there have been few studies of its adverse health effects.

Secondly, the prevalence of use of alcohol and tobacco is much higher than that of cannabis. For example, the proportions of the Australian population who are at least weekly users of alcohol, tobacco and cannabis are: 61 per cent, 29 per cent, (Department of Health, Housing and Community Services, 1992), and 11 per cent (Donnelly and Hall, 1994) respectively. Any overall comparison of the health consequences of the three drug types that was based upon existing patterns of use would unfairly disadvantage alcohol and tobacco. Any attempt to adjust for the differences in prevalence (e.g. by estimating the health effects if the prevalence of cannabis use was the same as those for alcohol and tobacco) would involve making controversial assumptions, so no such attempt has been made.

The very different prevalence of use of alcohol, tobacco and cannabis, and the fact that we know a great deal more about the adverse effects of alcohol and tobacco use, precludes any quantitative comparison of the current health consequences of these drugs. Nevertheless, a qualitative comparison of the probable health risks of cannabis with the known health risks of alcohol and tobacco serves the useful purpose of reminding us of the risks we currently tolerate with our favourite psychoactive drugs.

In undertaking this qualitative comparison, we have avoided the necessity to comprehensively review the vast literatures on the health effects of alcohol and tobacco by using the following authorities as the principal sources of evidence for our assertions about their health risks: Anderson et al (1993); Holman et al's (1988) compendium of the health effects of alcohol and tobacco; the Institute of Medicine (1987); the International Agency for Research into Cancer (1990); Roselle et al (1993); and the Royal College of Physicians (1987).

9.3.1 Acute effects

Alcohol. The major risks of acute cannabis use are similar to the acute risks of alcohol intoxication in a number of respects. First, both drugs produce psychomotor and cognitive impairment, especially of memory and planning. The impairment produced by alcohol increases risks of various kinds of accident, and the likelihood of engaging in risky behaviour, such as dangerous driving, and unsafe sexual practices. It remains to be determined whether cannabis intoxication produces similar increases in accidental injury and death, although on balance it probably does.

Second, there is good evidence that substantial doses of alcohol taken during the first trimester of pregnancy can produce a foetal alcohol syndrome. There is suggestive but far from conclusive evidence that cannabis used during pregnancy may have similar adverse effects.

Third, there is a major health risk of acute alcohol use that is not shared with cannabis. In large doses alcohol can cause death by asphyxiation, alcohol poisoning, cardiomyopathy and cardiac infarct. There are no recorded cases of fatalities attributable to cannabis, and the extrapolated lethal dose from animal studies cannot be achieved by recreational users.

Tobacco. The major acute health risks that cannabis shares with tobacco are the irritant effects of smoke upon the respiratory system, and the stimulating effects of both THC and nicotine on the cardiovascular system, both of which can be detrimental to persons with cardiovascular disease.

9.3.2 Chronic effects

Alcohol. There are a number of risks of heavy chronic alcohol use, some of which may be shared by chronic cannabis use. First, heavy use of either drug increases the risk of developing a dependence syndrome in which users experience difficulty in stopping or controlling their use. There is strong evidence of such a syndrome in the case of alcohol and reasonable evidence in the case of cannabis. A major difference between the two is that it is uncertain whether a withdrawal syndrome reliably occurs after dependent cannabis users abruptly stop their cannabis use, whereas the abrupt cessation of alcohol use in severely dependent drinkers produces a well defined withdrawal syndrome which can be potentially fatal.

Second, there is reasonable clinical evidence that the chronic heavy use of alcohol can produce psychotic symptoms and psychoses in some individuals. There is suggestive evidence that chronic heavy cannabis use may produce a toxic psychosis, precipitate psychotic illnesses in predisposed individuals, and exacerbate psychotic symptoms in individuals with schizophrenia.

Third, there is good evidence that chronic heavy alcohol use can indirectly cause brain injury - the Wernicke-Korsakov syndrome - with symptoms of severe memory defect and an impaired ability to plan and organise. With continued heavy drinking, and in the absence of vitamin supplementation, this injury may produce severe irreversible cognitive impairment. There is good reason for concluding that chronic cannabis use does not produce cognitive impairment of comparable severity. There is suggestive evidence that chronic cannabis use may produce subtle defects in cognitive functioning, that may or may not be reversible after abstinence.

Fourth, there is reasonable evidence that chronic heavy alcohol use produces impaired occupational performance in adults, and lowered educational achievements in adolescents. There is suggestive evidence that chronic heavy cannabis use produces similar, albeit more subtle impairments in occupational and educational performance of adults.

Fifth, there is good evidence that chronic, heavy alcohol use increases the risk of premature mortality from accidents, suicide and violence. There is no comparable evidence for chronic cannabis use, although it is likely that dependent cannabis users who frequently drive while intoxicated with cannabis increase their risk of accidental injury or death.

Sixth, alcohol use has been accepted as a contributory cause of cancer of the oropharangeal organs in men and women. There is suggestive evidence that chronic cannabis smoking may also be a contributory cause of cancers of the aerodigestive tract.

Tobacco. The major adverse health effects shared by chronic cannabis and tobacco smokers are chronic respiratory diseases, such as chronic bronchitis, and probably, cancers of the aerodigestive tract (i.e. the mouth, tongue, throat, oesophagus, lungs). The increased risk of cancer in the aerodigestive tract is a consequence of the shared route of administration by smoking. It is possible that chronic cannabis smoking also shares the cardiotoxic properties of tobacco smoking, although this possibility remains to be investigated.

It should be stressed that this section only describes the adverse health effects of alcohol and tobacco for which there is some evidence that chronic heavy cannabis use may also cause. It does not, therefore, provide an exhaustive inventory of all the adverse health effects of either chronic alcohol or tobacco use. Among the major additional adverse health effects of chronic heavy alcohol use which are not shared by cannabis are: liver cirrhosis, peripheral neuropathy, and gastritis.